Image: NASA
Clouds over the Indian ocean, observed with the instrument MODIS at the NASA satellite Aqua. Photo: NASA

Climate models currently include the global warming effect of greenhouse gases as well as the cooling effects of atmospheric aerosols. The tiny particles that make up these aerosols are produced by human-made sources such as emissions from cars and industry, as well as natural sources such as phytoplankton and sea spray.

Aerosols directly influence flow of sunlight and heat

They can directly influence the flow of sunlight and heat within the Earth’s atmosphere as well as interact with clouds. One of the ways that they do this is by bolstering clouds’ ability to reflect sunlight back into space by increasing their droplet concentration. This in turn cools the planet.  The amount of sunlight that is reflected to space is referred to Earth’s albedo.

However, there has been extremely limited understanding of how aerosol concentration has changed between early-industrial times and the present day. This lack of information restricts the ability of climate models to accurately estimate the long-term effects of aerosols on global temperatures –and how much of an effect they could have in the future.

New study in PNAS

Now, an international study led by the Universities of Leeds and Washington and with Frida Bender at the Department of Meteorology, Stockholm University as co-author, has recognised that remote, pristine parts of the Southern Hemisphere provide a window into what the early-industrial atmosphere looked like.

The team used satellite measurements of cloud droplet concentration in the atmosphere over the Northern Hemisphere — heavily polluted with today’s industrial aerosols — and over the relatively pristine Southern Ocean.

They used these measurements to quantify the possible changes due to industrial aerosols in Earth’s albedo since 1850.

The results, published in the journal PNAS, suggest that early-industrial aerosol concentrations and cloud droplet numbers were much higher than is currently estimated by many global climate models. This could mean that human-generated atmospheric aerosols are not having as strong a cooling effect as some climate models estimate. The study suggests that the effect is likely to be more moderate. 

The paper “The hemispheric contrast in cloud microphysical properties constrains aerosol forcing” is published in PNAS, 20 July 2020 (DOI: 10.1073/pnas.1922502117)

Read more at University of Leeds website.